iOS -- CoreData data migration

Coredata provides a lightweight auto data migration method for the following conditions:

1. Simply add a new column.
2. Change a column to optional.
3. Change a optional column to compulsory, but have default value.

Step 1:
Create a new data version


1. Editor -> Add Model Version (expand your xcdatamodeld item) , type a name

2. Client on the xxxx.xcdatamodeld file and in the attribute page, set the current version



Step 2:

1. In App delegate , in method "persistentStoreCoordinator" change as follow:

- (NSPersistentStoreCoordinator *)persistentStoreCoordinator {
if (__persistentStoreCoordinator != nil) {
return __persistentStoreCoordinator;
}

NSURL *storeURL = [[self applicationDocumentsDirectory] URLByAppendingPathComponent:@"WorkXP.sqlite"];

NSError *error = nil;
__persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]];

// handle db upgrade
NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithBool:YES], NSMigratePersistentStoresAutomaticallyOption,
[NSNumber numberWithBool:YES], NSInferMappingModelAutomaticallyOption, nil];

if (![__persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeURL options:options error:&error]) {
// Handle error
} 

return __persistentStoreCoordinator;
}


After that, for a lightweight data migration, user don't need to remove the old version app before they install the new one.